LAND PLANTS
I. Origin.
Earth Chronology (these dates are not exact):
4.6 billion years ago: the earth formed.
4-3.8 billion years ago: life originated.
3.8 billion years ago: prokaryote anaerobes, heterotrophs.
3.5 billion years ago: oldest known fossils: microfossils and stromatolites; photosynthesis.
2.5 billion years ago: photosynthesis established, oxygen accumulated.
1.5 billion years ago: first eukaryotes.
700 million years ago: soft-bodies multicellular life.
540 million years ago: hard-bodied multicellular life.
476 million years ago: first evidence of land plants.
The colonization of land by plants occurred before 476 million years ago, during the Ordovician.
Land plants are probably derived from a group of green algae called charophytes.
II. Phylogeny
Definition.
Phylogeny studies the evolutionary history of a taxon: its origin and evolution.


The description and explanation of the changes of a taxon that have occurred over
millions of years.
These changes are morphological, physiological, ecological, and biogeographical.
The fossil timeline.
The evolution of land plants can arbitrarily be divided into five stages or interval of time.
Remember that evolution is a continuum and these intervals or evolutionary stages are
presented here in order to facilitate learning.
a) Origin of land plants.
First fossil evidence of land plants appeared about 476 million years ago and lasted about 60
million years.
Three types of microscopic fossils have been found so far: spores, waxy cuticle and small
tubes.
The substance sporopollenin was probably what allowed these parts to resist decay.
b) Diversification.
In a relatively short time of about 60 million years, plant diversified abundantly and colonized
many land areas.
Between 410 and 360 million years ago, the Silurian-Devonian Diversification Explosion took
place.
First roots, leaves, vascular tissue, wood, etc. appear in the fossil record.
c) Primeval forests.
First evidence of forests appears during the Carboniferous period, between 350 and 290 million
years ago.
Extensive deposits of coal packed with fossils of spores, trunks, wood, leaves, etc.
Extensive forested swamps.
d) The seed.
This interval is dominated by seed plants called gymnosperms or naked-seed plants.
This interval lasted from about 250 to 120 million years ago, from the late Permian to the early
the Cretaceous.
Modern gymnosperms include pines, spruces, firs, cycads and ginkgoes among others.
Gymnosperms prefer dry habitats so paleontologists theorize that during this time, plants moved
into dryer areas away from the swamps. Both wet and dry land habitats became colonized.
e) Flowering plants or angiosperms.
From the early Cretaceous to the present, the last 145 million years approximately.
Flowers and covered seeds evolved.
The evolution of plants is still going on.
A Phylogeny of Land Plants.
Paul Kenrick and Peter Crane collated morphological and molecular data to produce a
phylogenetic tree that shows possible relationships between the different groups of land
plants.
See Fig. 28.2 on page 543.
Some important points to consider about their phylogenetic tree.

The oldest branches of the tree lead to a group of algae called "green algae."
Land plants share with the green algae the following traits:
1. Chlorophyll a and b, xanthophylls (yellow carotenoids) and carotenes (orange
carotenoids).
2. Store carbohydrates in the form of starch.
3. Cell wall made mostly of cellulose.
4. Details of the formation of the cell plate.
5. DNA and RNA sequences support their close relation to the charophytes.

The ancestor of land plants probably came from a group of green algae called
charophytes.

Liverworts, hornworts and mosses are the earliest land plants.

The Rhyniopsida, are the first group to show water-conducting cells called tracheids and
are leafless.

The lycopods, horsetails and ferns have vascular tissue, true leaves and reproduce by
spores.

Gymnosperms and angiosperms are the more recent groups. They have seed, complex
leaves and have vascular tissue.
Present day charophytes inhabit ponds and lakes. This supports the hypothesis that transition to
land occurred from fresh water rather than salt water habitats.
III. Adaptations to terrestrial environments.
Desiccation or drying-out was a major adaptive problem to be solved in the conquest of the dry
land.
A transport system of water from the source, soil, to parts of the plant body had to be
developed.
Reproduction without the use of water to transport gametes presented another problem.
Upright support was a third obstacle to be overcome.
Preventing water loss:
1. Waxy cuticle to protect against desiccation.
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The cuticle is a waxy layer that covers many parts of the plant body.
It is impermeable to water and prevents loss and absorption of water by the covered
surfaces.
The cuticle also prevents gas exchange.
2. Stomata for gas exchange and control of transpiration.
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The stoma (plural stomata) is an opening surrounded by specialized cells called guard
cells.
The stoma opens or closes as the guard cells change shape. When the guard cells
become turgid, the stoma opens; when the guard cells lose water and become soft, the
stoma closes.
The opening and closing of the stomata allows plants to regulate gas exchange and
water loss.
The Rhyniopsida that lived about 385 million years ago, are the first plants to show
stomata.
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Hornworts and mosses have stomata with guard cells.
Liverworts have pores without guard cells.
Transporting water.
1. Upright position.
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Aquatic plants are supported by water. Water is 1000 times denser than air. Air is not as
buoyant.
400 million-year old fossils show that those plants grew upright.
Cell wall contains lignin, a polymer, to strengthen and support upright structures.
This adaptation appeared in several plant lineages.
2. Transport system or vascular tissue:


Phloem for the transport of dissolved carbohydrates.
Xylem for water and mineral transport.
There are two modern type of cells specialized to transport water and solutes:
Tracheids
 Dead at maturity.
 Long and tapered cells.
 Have secondary walls except at pits.
 Pits on lateral and end walls.
 Conduction of water and minerals.
 Appeared about 380 million years ago.
Vessel elements
 Dead at maturity.
 Long and cylindrical cells joined end to end.
 Secondary cell wall except at pits.
 End walls have perforations.
 Conduction of water and minerals.
 Appeared about 250-270 million years ago.
Transporting gametes and protecting the embryo.
1. The multicellular haploid phase called the gametophyte.
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Gametohytes produce gametes by mitosis.
Fusion of gametes produces a diploid sporophyte.
Multicellular gametangia made of a layer of sterile cells to protect gametes:
 Antheridia produce sperms.
 Archegonia produce eggs.
After fertilization, the egg develops into a multicellular embryo within the
archegonium.
Alternation of generations.
2. The multicellular diploid phase is called the sporophyte.
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Sporophytes produce spores by meiosis.
Germination of haploid spores produces a haploid gametophyte.
Alternation of generations.
3. The flower.
Appeared about 145 million years ago during the early Cretaceous.
The flower is a modified branch apex, and is involved in sexual reproduction.
Reproductive and accessory organs are normally arranged in whorls or circles of structures:

Sepals, petals stamens, and carpels.
The flower contributed to the success of angiosperms and their present dominant status.
Coevolution of flower morphology and pollinators' behavior and anatomy contributed to more
effective pollination and seed formation thus increasing the angiosperm reproductive
advantage over other plants including the seed-producing gymnosperms.
Pollination is the transfer of pollen from the anther to the stigma, part of the female organ of the
flower.
IV. The Calvin-Benson Cycle
Plants have become adapted to capture light energy under different terrestrial environments.
C4 Photosynthesis
Review the Calvin Cycle of photosynthesis beginning on page 146
Two separate groups of scientists lead by H. Kotschack and Y. S. Karpilov found that
radioactive carbon given to plant in carbon dioxide (14CO2) was incorporate in 4-carbon organic
acids like malate and aspartate, rather than in the expected 3-phosphoglycerate (3PG)
produced at the beginning of the Calvin Cycle.
Hatch and Slack continued the research and found that the plants used by the previous two
groups of scientists utilize two enzymes to fix carbon dioxide:
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Rubisco found in the bundle-sheath cells surrounding the vascular tissue of the leaf.
PEP carboxylase found in the mesophyll cells of the leaf.
PEP carboxylase has greater affinity for CO2 than rubisco and can fix CO2 when
rubisco cannot.
Hatch and Slack proposed a model to explain how the carbon fixed in the 4-C organic acids is
used in the Calving Cycle.
CO2 is fixed by PEP carboxylase in a 3-C compound called phosphoenolpyruvate, PEP, and
form a 4-C organic acid. This occurs in the mesophyll of the leaf.
The 4-C malate or aspartate then travels to the bundle-sheath cells and released the CO2 in a
reaction catalyzed by rubisco that forms 3C phosphoglycerate, 3PG. The Calvin-Benson Cycle
continues to form carbohydrates.

The 4C organic acids travel to the bundle-sheath cells through plasmodesmata.
Why this pathway?
In C3 the first organic compound formed through the fixation of CO2 is 3PG.
When conditions are hot and dry the stomata close and the concentration of CO2 inside the cell
to decrease and the concentration of O2 to rise - why?
The low level of carbon dioxide starves the Calvin Cycle.
Under low CO2 and high O2, rubisco prefers the oxygen and adds oxygen to the Calvin Cycle.
The product of the addition of oxygen results in the splitting of the molecule.
One of the products is a 2C molecule that is exported from the chloroplasts and broken down in
the mitochondria and peroxisomes to form CO2.
This process is called photorespiration.
Photorespiration utilizes organic material that normally would be used in the Calvin Cycle and
produces no food.

In some plants, photorespiration utilizes up to 50% of the carbon dioxide fixed by the
Calvin cycle.
Photorespiration decreases photosynthetic output.
But in C4 plants...
The mesophyll cells of C4 plants pump enough carbon dioxide inside the bundle-sheath cells
keeping the concentration high enough for rubisco to accept CO2 rather than O2.
This minimizes photorespiration and increases sugar production.
This adaptation is advantageous in hot and dry climates with intense sunlight where plants must
close the stomata to conserve water.
Crassulacean Acid Metabolism or CAM
This pathway was discovered in the members of the Crassulaceae family of plants.
Like the C4 pathway, CAM increases the concentration of CO2 inside the cell.
CAM occurs in plants that inhabit climates that are hot and dry and keep their stomata closed
during the day, e. g. cacti.
CAM plants close their stomata during the day in order to conserve water but this prevents
carbon dioxide from entering the leaf.
CAM plants open their stomata during the night when conditions are less hot and fix large
amounts of CO2 in organic acids that are stored in vacuoles.
During the day, the stomata close but sunlight is then utilized to carry on the light-dependent
reactions.

Sunlight is used to supply ATP and NADPH for the Calvin cycle.
During the day, the organic acids stored in the vacuoles are broken down to release carbon
dioxide to be fixed in the Calvin cycle, in the light-independent reactions.
C4 and CAM pathways are similar in that carbon dioxide is incorporated into organic acids in
both.
But they differ in location and time:


in C4 the fixation of CO2 is separated structurally from the Calvin cycle.
In CAM the fixation of CO2 and the Calvin cycle occur in the same cell but at different
times: night and day.
These two pathways allow angiosperms to colonize very dry habitats.
V. Human uses of plants.
Man in different parts of the world domesticated plants at different times.
Domestication of wild plants began about 10,000 years ago.
Man selected plants consciously or unconsciously for his own benefit.
As wild forms were brought under cultivation, some changes occurred: seeds became larger,
fruits became sweeter, plant parts became non-poisonous, etc.
Plants are at the base of the food chain.
Plants are also utilize to make coal, firewood, clothing, paper
Plant biologists are using DNA techniques to transfer genes to create plant varieties more
suitable to a particular end.
Chapter 29
PLANT DIVERSITY I:
HOW PLANTS COLONIZED LAND
There are about 290,000 of land plants on Earth.
LAND PLANTS EVOLVED FROM GREEN ALGAE
Charophycean ancestor
Charophyceans are the green algae most closely related to land plants.
Land plants probably are probably derived from a group of green algae called charophytes.
Land plants share with the charophyceans the following traits:
1. Rosette cellulose-synthesizing complexes: land plants and charophyceans posses a
rosette-shape array of proteins that synthesize cellulose microfibrils in their cell wall.
Other cellulose wall-containing algae (e. g. brown algae, dinoflagellates), have linear arrays
of cellulose-producing proteins.
This suggests a common ancestor between the charophytes and land plants. This rosette
synthesizing system evolved independently of the cellulose making system of other green
algae.
2. Peroxisomes enzymes: the charophyceans and land plants have enzymes in their
peroxisomes that minimize the loss of carbohydrate due to photorespiration.
Other alga groups do not have these enzymes in their peroxisomes.
3. Structure of the flagellate sperm: details of the sperm of charophyceans resemble those
of land plants that have flagellated sperms.
4. Cell plate formation during cytokinesis: cell division features a complex network of
microtubules and Golgi vesicles, the phragmoplast, again as found in all land plants.
5. DNA and RNA sequences support their close relation to the charophytes, especially Chara
and Coleochaete.
TERRESTRIAL ADAPTATION OF LAND PLANTS
ADAPTATIONS ENABLING THE MOVE TO LAND
A layer of sporopollenin protects charophytes from desiccation; sporopollenin is found in the
spore wall of land plants.
Danger of desiccation required new adaptations: transport tissue, cuticle, etc.
Support against gravity.
Plants are eukaryotic, multicellular, mostly autotrophic organisms, with haploid-diploid life
cycles, which retain embryo within female sex organ on parent plant; the cell wall contains
cellulose.
Scientists are studying the ultrastructure of cells, analyzing macromolecules and comparing
morphology with life cycles.
There are several proposals to rearrange the boundaries of the kingdom Plantae:

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
Only the Embryophytes; the present and traditional system.
Expand it to include the charophyceans: Kingdom Streptophyta.
Expand it further to include all the green algae, Chlorophyta: Kingdom Viridiplantae.
DERIVED TRAITS OF PLANTS
The following characteristics are common to all four groups of land plants but are absent in the
charophyceans.
1. Apical meristem: cluster of embryonic cells found at the tip of shoots and roots.
2. Alternation of generations: a characteristic life cycle.
Alternation of generation does not occur in the charophyceans. This suggests that
alternation of generation arose independently in land plants. A life cycle characterized by a
multicellular haploid gametophyte stage followed by a multicellular diploid sporophyte
stage.
Multicellular, dependent embryos: The zygote is retained surrounded by tissues of the
gametophyte. The parental tissue provides the embryo with nutrients. Placental transfer
cells present in the embryo and sometimes in the gametophyte as well, enhance the
transfer of nutrients.
3. Spores produced in sporangia: haploid reproductive cells that become a multicellular
haploid gametophyte by mitosis.
The multicellular sporangium contains sporocytes, the cells that undergo meiosis to form
spores.
Sporopollenin, the most durable organic material known, makes the walls of the spores.
4. Multicellular gametangia: the gametes of land plants are produced in multicellular organs
called gametangia. Algae produce their gametes in unicellular gametangia, inside a single
cell.
Adaptations for water transport and conservation.
3. Waxy cuticle to protect against desiccation.
4. Stomata (sing. stoma) for gas exchange and control of transpiration.
5. Transport system or vascular tissue
Secondary metabolic compounds
Land plants make many metabolic compounds that are produced by side branches off the
primary metabolic pathways that make lipids, carbohydrates, proteins and other compounds
common to all organisms.
Cell wall contains lignin, a polymer, to strengthen and support upright structures.
Other secondary compounds are alkaloids, tannins, and phenolics (flavonoids).
These compounds functions as a protection against herbivores, absorb harmful UV radiation,
and are involved in the symbiotic relationship with soil microbes.
ORIGIN OF LAND PLANTS
About 475 million years ago, in the mid-Ordovician, plants were widespread all over the world
as shown by the many spores found in sediments of this period.
In a relatively short time of about 50 million years, plant diversified abundantly and colonized
many land areas.
There are four main groups of land plants:
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Bryophytes, including mosses.
Pteridophytes, including ferns and seedless vascular plants.
Gymnosperms, including conifers.

Angiosperms including flowering plants.
Land plants are distinguished from algae by the production of multicellular embryos that remain
attached to the mother plant, which protects and nourishes the embryos.
Bryophytes are distinguished from the other three groups of land plants by the lack of a vascular
tissue made of special cells called xylem and phloem. Some bryophytes have water and nutrient
transport system made of a different kind of cells.

The vascular system transports water and nutrients.
Pteridophytes do not produce seeds. Gymnosperms and angiosperms produce seeds.
A seed consists of a plant embryo with a food storing tissue and a surrounding coat for
protection.
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The first vascular plants to produce seeds evolved about 360 million years ago.
Their seeds were not enclosed in any specialized chamber.
Angiosperms produce flowers and conifers produce "cones", a specialized reproductive
structure.
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Angiosperms produce their seeds in specialized chambers called ovaries.
Gymnosperms do not produce seed in ovaries.
The word grade is used to designate a collection of organisms that shate a common level of
biologial organization or adaptation.
BRYOPHYTES
About 17,000 species worldwide divided into three Divisions or phyla: Bryophyta, the mosses;
Hepatophyta, the liverworts; and Anthocerophyta, the hornworts.
Their life cycle is similar but the three groups may not be closely related. The bryophytes may
form a polyphyletic group.
Bryophyta refers to the phylum of mosses only; bryophytes refer to the three phyla mentioned
above.
Characteristics of the bryophytes:
1. Small plants found in moist environments, lack woody tissue and usually form mats spread
over the ground.
2. Gametophyte generation is dominant; sporophyte is parasitic on the gametophyte.
3. Bryophytes have cuticle, stomata and multicellular gametangia that allow them to survive on
land.
4. Bryophytes need water to reproduce and most species lack vascular tissue (xylem and
phloem).
5. Water transport is mostly through capillary action, diffusion and cytoplasmic streaming. They
lack true roots, stems and leaves.
The gametophyte of mosses is a one-cell-thick filament known as the protonema that eventually
produces buds having meristematic tissue. These meristems produce an upright structure called
the gametophore.
These gametophytes are one to a few cells thick and obtain nutrients and water by direct
absorption from the environment.
Most mosses do not have conducting tissue. Some species have specialized cells that conduct
water and nutrients but lack lignin in their cell walls.
The gametophores are anchored by fragile rhizoids. Rhizoids are either single elongated cells
as those found in liverworts and hornworts, or filaments of cells as those of mosses.
Rhizoids are not made of tissues and do not absorb any significant amount of water. In that way
they differ from roots.
Bryophytes have smallest and simplest sporophyte of any group.
The sporophyte remains attached to the gametophyte throughout its lifetime, dependent of the
gametophyte for food, water and minerals.
The mature sporophyte of mosses consists of a foot embedded in the archegonium, a seta or
stalk is present in the phylum Bryophyta, and a capsule or sporangium. The cap or calyptra
closes the peristome or opening or the capsule.
THE ORIGIN OF VASCULAR PLANTS.
Ferns and other seedless vascular plants formed the first forests.
1. The next step in land plant evolution included the development of an independent
sporophyte.
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At first this sporophyte was of equal size as the gametophyte.
Cooksonia caledonica, from the Silurian (~420 million years ago) rocks of Europe and
North America, is the oldest known land plant.
Small, leafless, rootless, dichotomous axes with terminal sporangia.
2. Transport in xylem and phloem.
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Phloem for the transport of dissolved carbohydrates.
Xylem for water and mineral transport.
Lignin strengthens the vascular tissue cells.
3. Evolution of roots.
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Roots anchor plants and allow the absorption of water and nutrients from the soil.
Root tissues of living plants closely resemble stem tissues of early vascular plants
preserved in fossils.
Roots may have evolved from the lowest subterranean parts of the stem.
The oldest lycophyte fossil had simple roots 400 million years ago.
4. From an evolutionary perspective, there are two kinds of leaves:
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Microphylls are single veined leaves associated and evolved as superficial
outgrowth of the stem.
Microphylls first appear in the fossil recorda about 410 million years ago.
Megaphylls have a complex venation pattern, and evolved from a branch system.
Megaphylls appeared about 370 million years ago, at the end of the Devonian.
5. The sporophyte became the dominant generation.
Sporophylls are modified leaves that bear spores.
Sporophylls may be grouped into cone-like structures called strobili (sing. strobilus).

Homospory: production of one kind of spores.
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Spores produce a bisexual gametophyte that produces eggs and sperms.
Heterospory: production of two kinds of spores.
-
Haploid megaspores develop into a female gametophyte.
Haploid microspores develop into a male gametophyte.
CLASSIFICATION OF SEEDLESS VASCULAR PLANTS
There are two phyla of pteridophytes found in the modern flora: Licophyta and Pterophyta.
1. Phylum Lycophyta.
There are about 15 genera of lycophytes and approximately 1000 living species.
This phylum includes the Lycopods (club mosses), Selaginella (spike moss) and Isoetes
(quillwort).
This evolutionary line extends back into the Devonian (409-363 mya) but were most prevalent in
the wet swamps of the Carboniferous period (363-290 mya).
They eventually split up into two evolutionary lines.
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The first were very large woody trees that did not survive in the drier climate at the end
of and after the Carboniferous age. In the Carboniferous some lycophytes were forestforming trees more than 35 meters tall.
The second and the surviving group of Lycopods are the small and herbaceous trees.
Lycophyta remains became the largest coal deposits of all geologic time.
The sporophytes of lycophytes consist of true roots, stems and leaves (microphylls).
Some Selaginella are heterosporous; Lycopodium is homosporous.
Sporophylls are specialized leaves that bear sporangia and are organized into a structure
called the strobilus (pl. strobili).
2. Phylum Pterophyta
Psilophytes (whisk ferns)
It includes two living genera, Psilotum and Tmesipteris, from tropical and subtropical regions of
the world...
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Sporophyte with a dichotomously branching aerial and subterranean stem system.
True roots lacking.
Underground stems with rhizoids and with a fungal association.
Aerial stems lacking leaves but with scale-like or larger leaf-like structures (enations)
Until recently they were placed in a phylum of their own, but DNA sequences analysis and
sperm ultrastructure study has shown that they are related to present day fern.
The lack of roots and leaves may be due to simplification, a derived or secondary characteristic,
rather than a maintained characteristic from ancient ancestors, a primitive characteristic.
Sphenophytes (Horsetails)
Sphenopsids extend back to the Devonian (409-363 mya) and reached their maximum
development in the Carboniferous (363-290 mya.)
A family of one extant genus, Equisetum (ca. 15 species), of nearly worldwide distribution in
damp habitats such as riverbanks, lakeshores, and marshes. Michigan is a center of diversity
for the genus with nine native species.
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The sporophyte of Equisetum is differentiated into an underground rhizome that bears
adventitious roots and an upright, photosynthetic stem with whorls of microphylls...

Tough perennial herbs with jointed, ridged aerial stems with distinct nodes.

Stems rough, accumulating silica and metals, and complex anatomically.

The aerial stems contain a large central pith region, which in mature plants is hollow.

Surrounding the pith cavity are discrete bundles of vascular tissue; this arrangement of
conducting tissue is known as a eustele.
Recent molecular data suggest that they are closely related to ferns and should be classified
with them.
Ferns
The fossil record of ferns extends back into the Carboniferous (363-290 mya) but their origins is
in the Devonian (409-363 mya).
There are about 12,000 species of ferns in the world. Most species are tropical.
 There are about 380 species of ferns in U.S.A. and Canada.
 Costa Rica has over 1,000 species.
 Philippines has over 950 species.
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Sporophyte is differentiated into true roots, stem (rhizome) and leaves (megaphylls).

Leaves usually differentiated into Stipe (petiole) and blade with a central rachis or vein.

Most ferns are homosporous; a few aquatic genera are heterosporous.

The sporangia are produced in clusters called sori (sing. sorus.) the sori can be arranged in
various patterns, e. g. rows or lines.
COAL FORESTS
The Lycophyta and Pterophyta represent the modern lineages of seedless vascular plants that
formed forests during the Carboniferous period about 290-363 million years ago.
The coal beds, oil fields and natural gas deposits that are mined in modern times are derived
from these ancient forests. From there comes the name fossil fuels.
During the Carboniferous Europe and North America were closer to the equator and covered
with extensive swamps. As plants died, their body did not completely decay in the stagnant
water and great depths of organic material accumulated forming peat.
These layers of peat were later covered by sediments that pressed the peat. Pressure and heat
converted the peat into coal, petroleum and gas.
Chapter 31
PLANT FORM AND FUNCTION
There are about 262,000 species of plants. About 235,000 species or 90%, are angiosperms.
Angiosperms can be either herbaceous or woody.
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Herbaceous plants can be annuals, biennials and perennials.
Woody plants are perennials.
The plant body consists of a root system and a shoot system.
I. ROOT AND STEM SYSTEMS
DIVERSITY OF ROOTS
Roots show great diversity of structure but their function is similar.
Principal function of roots: support, storage and absorption.
Roots absorb many nutrients from the soil.
N, K and P are the limiting nutrients that plants need and often are depleted from the soil.
Roots may vary from long, deep, taproots to shallow, fibrous masses of roots. Adventitious roots originate
in organs that are not the primary root, e.g. leaves, branches.
DIVERSITY OF STEMS
Stems show great variety of structures: trunks, creeping stems, rosettes, tubers, rhizomes, etc.
Principal function of stems:
1. Support of the photosynthetic, reproductive and storage parts.
2. Conduction of water and metabolites.
3. Production of new stem tissues.
Different structure (shape, size, etc.) of stems and roots help plants survive in specific habitats.

Stem variation is structure is adaptive.
Closely related species and families of plants tend to have similar morphology and anatomy inherited
from a common ancestor.
II. CELLS, TISSUES, ORGANS AND SYSTEMS.
Plants are made of cells organized into tissues and organs.

Roots, stems, flowers and fruits are organs.
There are three tissue systems that extend throughout the entire body of the plant.
Each tissue system contains two or more tissues, which can be simple or complex depending on the
kinds of cells that form the tissue.
CELLS
Parenchyma cells
 Living cells at maturity.
 Have thin primary walls.
 Function in storage, secretion and photosynthesis.
 Found throughout the body of the plant.
Collenchyma cells
 Living cells at maturity.
 Have unevenly thickened primary cell walls.
 Function in support in flexible parts of the plant.
 Found in petioles, leaf veins and other parts of the plant that must be flexible.
Sclerenchyma cells
 Have both primary and thickened secondary cell walls.
 Cells are often dead at maturity.
 Secondary wall with pits.
 Provide structural support.
Tracheids
 Dead at maturity.
 Long and tapered cells.
 Have secondary walls except at pits.
 Pits on lateral and end walls.
 Conduction of water and minerals.
Vessel elements
 Dead at maturity.
 Long and cylindrical cells joined end to end.
 Secondary cell wall except at pits.
 End walls have perforations.
 Conduction of water and minerals.
Sieve tube members
 Living cells at maturity.
 Lack nucleus and other organelles at maturity.
 Elongated cells, cylindrical, joined end to end.
 Secondary cell wall present.
 End walls are sieve plates with holes.
 Cytoplasm extends from one cell to the next through the holes of the sieve plate.
 Conduct the products of photosynthesis.
Companion cells
 Living cells at maturity.
 Associated to a sieve tube members by means of plasmodesmata.
 Assists in moving sugars in and out of sieve tube members.
 The nucleus is thought to direct the activity of both cells.
TISSUES
1. Epidermal tissue consists of a single layer of cells, the epidermis, which covers the entire body of
the plant except the woody areas where cork tissue develops.
2. Cork tissue or periderm consists of cork cells, cork cambium and cork parenchyma.
3. Ground tissue is located below the epidermis and surrounds the vascular tissue. It is made up of
parenchyma cells often strengthened with collenchyma and sclerenchyma cells.

Photosynthesis, storage, secretion, flexible and rigid structural support.
4. Vascular tissue is complex. It is made of two tissues:
Xylem
tracheids
vessel elements
parenchyma cells
fibers
Phloem
sieve tube members
companion cells
parenchyma cells
fibers

The function of the xylem is to transport water and nutrients from the roots to other parts of
the plant.

The function of the phloem is transport photosynthates from the green areas of the plant to
the living cells found in the roots, stems, etc.
III. ANATOMY.
Growth in plants is localized in regions called meristems.
ROOT ANATOMY
The root has three concentric tissues: epidermal, ground and vascular tissues.
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The vascular tissue forms a central cylinder called the stele.
Secondary roots arise from a layer of cells in the stele called the pericycle, and grow horizontally
until they erupt through the epidermis.
The epidermis has root hairs used in absorption.
Root structure is different from that of the stem. It differs in.

Root cap, a protective layer that covers the apical meristem and orients the root to grow
downward. Root cap responds to gravity (gravitropism).

Root hairs, short-lived extensions of the epidermal layer, to increase absorption.

Roots have a pericycle and endodermis present.
Roots lack nodes and internodes.
STEM ANATOMY
External morphology
1. Terminal and lateral buds.
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Covered with bud scales while dormant.
Contain a growth cluster of cells called meristem and produce new or primary tissues.
Lateral buds are associated with leaf axils.
Bud scale scars.
2. Nodes and internodes.
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
Nodes are the regions of leaf attachment.
Internodes are the space between two nodes.
3. Leaf scars and bundle scars.
4. Lenticels.
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Loosely arranged cells that allow gas exchange.
Broken epidermis.
LEAF ANATOMY
External morphology
Leaves are organs and vary greatly in external form.
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They may range in length from about 20 m (65 ft) to about 0.15 cm (0.06 in).
Their principal parts are blade, petiole and stipules (may be absent).
Leaves can be simple or compound.
Their arrangement along the stem can be alternate, opposite or whorled.
Their venation may be netted or parallel.
Veins can be arranged pinnately or palmately
Anatomy
1. Epidermis
There are upper and lower epidermises that form the surface of the leaf.
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Made of living parenchyma cells.
Lack chloroplasts.
Covered with a waxy layer, the cuticle.
Have small openings for gas exchange called stomata (sing. stoma).
Guard cells flank each stoma.
Trichomes or hairs may be present.
2. Mesophyll
The photosynthetic tissue found in between the two epidermises is called mesophyll.
It consist of...
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Made of living parenchyma cells.
Abundant chloroplasts.
Usually loosely arranged with many air spaces.
Often arrange in two regions: palisade and spongy mesophyll.
3. Venation
Veins and diffusion cooperate in the movement of materials in veins.
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Veins or vascular bundles extend through the mesophyll.
Each vein contains xylem and phloem tissue.
Xylem is usually restricted to the upper side of the vein, and phloem to the lower side of the vein.
A non-vascular parenchymatous layer of cells called the bundle sheath surrounds veins.
The bundle sheath extensions are support columns of cells that extend from the vein to the
upper and lower epidermis.
May be composed of parenchyma, collenchyma or sclerenchyma cells.
Monocot leaves usually have parallel venation and it is not differentiated into palisade and spongy
mesophyll; the guard cells are shaped like dumbbells
Dicot leaves have netted venation, mesophyll differentiated into two regions, and the guard cells are
bean-shaped.
Subsidiary cells are epidermal cells associated with the stomata and are involved in the opening and
closing of the stomata.
Gymnosperms normally have parallel or free venation (not netted).
GROWTH
Growth in plants is localized in regions called meristems.

It involves cell division, cell elongation and cell differentiation.
1. Primary growth causes the roots and stems to elongate.


It occurs in all plants.
The apical meristem at the tip of roots and stems is responsible.
2. Secondary growth is an increase in stem and root girth.
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It occurs in woody plants and a few herbaceous plants only.
It is due to the activity of the lateral meristems: vascular cambium and cork cambium.
Vascular cambium forms a cylinder along the length of roots and stems, between the xylem and
phloem; it produces more xylem and phloem.
Cork cambium is located in the outer bark.
Tree rings are formed by the alternation of periods of growth and dormancy.
Genetically identical individuals differ in response to environmental differences (e.g. growing in the shade
or in the sun). This is called developmental plasticity.
WATER AND SUGAR TRANSPORT
Chapter 32
In a hot summer day, an oak tree may lose up to 55 gallons of water in a single day.
Water loss due to transpiration is inevitable for plants. The stomata must be opened in order to
obtain carbon dioxide and release the oxygen produced during photosynthesis.
WATER POTENTIAL AND CEL-TO-CELL MOVEMENT
Osmosis is passing of solvent (e.g. water) molecules through a semipermeable membrane.
Water moves from the region of high water concentration to region of low water concentration.
Note that high concentration of water means low concentration of solutes.
The difference in solute concentration of different sides of a membrane is called the solute
potential or osmotic potential.
Water inside the cell pushes against the cell membrane, which in turns pushes against the cell
wall creating what is call, the turgor pressure.
The stiff wall produces a counteracting equal force against the turgor pressure called the wall
pressure.
Cells that are firm with these two forces acting against each other are said to be turgid.
The sum of these forces is called pressure potential.
Water potential
Water potential is the tendency of water to move from one location to another.
Water potential is a measure of the solute potential and pressure potential of the cell.
Water potential is expressed by the Greek letter Ψ (sigh).

Ψ = Ψ s + Ψp
Water potential is a form of potential energy.
Water potential is a measure of the cell's ability to absorb water by osmosis.
It also measures the water's tendency to evaporate from the cell.
Water potential of pure water is 0 by convention, because it has no solutes.
When there is a solute, the water potential is expressed with a negative sign because the water
potential is less than in pure water. The solutes make the water in the cell less likely to move
out.
The pressure potential (Ψp ) pushes on the water inside the cell and makes it more likely to
move out of the cell. The pressure potential carries a positive sign.
The energy of pure water is 0 megapascals ( 1 MPa = 10 atmospheres or 14.5 pounds/inch2).
When solutes dissolve in water, the free energy of water decreases and the water potential
becomes a negative number.
Water moves from the region of high water potential (very negative) to the region of lower water
potential (less negative).
When a cell loses water, the cell membrane separates from the cell wall and the cell becomes
flaccid. This state is called plasmolysis.
The effects of water movement.
Water potential can also be measured for tissues, organs and systems.
Water moves from tissues with high water potential to surrounding tissues with lower water
potential.
The air around the plant and the soil have water potential.
Water in the soil contains solutes and it can be under pressure.
The water potential of air, soil and plant tissues change constantly under the influence of wind,
sun, rain, etc.

The water potential in the soil is usually high relative to plant tissues.

The water potential of the air is usually low relative to plant tissues.
There is a series of water potential differences between the soil, plant tissues and air called the
water potential gradient.
TRANSPIRATION AND WATER MOVEMENT FROM ROOTS TO LEAVES.
The loss of water from leaves, stems and other aerial parts of the plant is called transpiration.
Cohesion-Tension Theory
Also known as the Transpiration-Cohesion Theory.
Water molecules are polar and attract each other. They form hydrogen bonds with adjacent
molecules.
Water molecules are also attracted by molecules other than water, e.g. molecules on the wall of
the tracheids and vessel elements.
This attraction by hydrophilic walls is called adhesion.
Mesophyll cells are surrounded by a film of water. This film of water lines the air spaces in the
mesophyll: the water-air interface.
At the water-air interface a meniscus is formed because the outer layer of water molecules are
pulled in only one direction while those below are equally pulled from several directions. The
meniscus is formed that way.
When water leaves the meniscus surface, the meniscus becomes more concave and its surface
tension increases.
Water is being pulled on by adhesive and cohesive forces creating a negative pressure. This
negative pressure pulls water molecules from places where the pressure is greater: the
cytoplasm of the mesophyll cells and xylem cells.
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

There is a gradient in water potential from the atmosphere down to the soil.
The atmosphere has very negative water potential.
Leaves have higher water potential than the atmosphere and lose water to it.
Stems have higher water potential than the leaves; the roots higher than the stem; and the
soil higher than the roots.
The gradient creates a pull of the column of water in the xylem due to the hydrogen bonds that
exist between the water molecules (cohesion).
Adhesion of the water molecules to the xylem walls maintains an unbroken column of water.
Root pressure pushes water from the root up the stem.
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
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Not strong enough to push the up tall plants.
It is very low or non-existent during the summer months.
Movement of water is greatest in the summer months when root pressure is the lowest.
According to the Cohesion-Tension Theory, plants do not spend energy in transpiration.
Limiting water loss.
How do plants cope with soils that have very low water potential?
1. Morphological adaptations that limit water loss.


Thick cuticle
Sunken stomata
2. Physiological adaptations that allow plants to function when water content is low.


CAM and C4 photosynthesis
Changes in solute potentials in roots
Increasing the concentration of solutes causes the solute potential to drop below the soil solute
potential causing water to move from higher to lower concentration (of water) following the water
gradient created by the solutes.
The anatomy of phloem tissue.
The phloem has two important cells, the sieve-tube elements and the companion cells.
Characteristics of these cells:
Sieve tube members
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Living cells at maturity.
Lack nucleus and other organelles at maturity.
Elongated cells, cylindrical, joined end to end.
Secondary cell wall present.
End walls are sieve plates with holes.
Cytoplasm extends from one cell to the next through the holes of the sieve plate.
Conduct the products of photosynthesis.
The most important photosynthate transported in the sieve-tube elements is sucrose, a
disaccharide.
Companion cells
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Living cells at maturity.
Associated to a sieve tube members by means of plasmodesmata.
Assists in moving sugars in and out of sieve tube members.
The nucleus is thought to direct the activity of both cells.
The Pressure-Flow Hypothesis.
Sucrose is the main sugar translocated in the phloem.
Sugars moves from the source where it is being produced, to the sink, where the sugars are
being utilized or stored.
This theory postulates that sugar moves in the phloem by means of a pressure gradient that
exists between the source, where sugar is loaded into the sieve tube members, and the sink,
where sugar is removed from the phloem.
1. Sucrose and other carbohydrates is actively loaded into the sieve tubes at the source.
Energy has to be spent in order to bring sugar into the sieve-tube elements against the
concentration gradient.
2. It requires ATP and a membrane transport system.
3. As a result water moves into the sieve tubes by osmosis increasing the hydrostatic pressure
in the sieve tubes.
4. Sugar is actively or passively unloaded from the sieve tube into tissues at the sink.
5. As a result water leaves the sieve tubes at the sink decreasing the hydrostatic pressure
inside the sieve tubes. Water is collected by the xylem and recycled.
6. A gradient is created between the source and sinks which drives the flow within the sieve
tubes.
Other substances transported in the phloem are hormones, ATP, amino acids, inorganic ions,
viruses and complex organic molecules like sugar-alcohol compounds.
Cotransport.
In cotransport, and ATP-powered system transports ions or molecules and indirectly powers the
movement of other solutes by maintaining a concentration gradient.

ATP is used to create a gradient of ions or molecules.
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The protein H+-ATPase hydrolyzes ATP into ADP and P, and uses the energy to
pump H+ across the membrane to the outside of the cell.
This establishes a difference in H+ and charge across the membrane. This difference
in charge is called an electrochemical gradient.
The tendency of the protons is to move back into the cell.
They do so through the opening provided by another protein called the
cotransporter.
When these ions move back to the lower concentration area in the cytoplasm, they
carry with it the molecules of sucrose against the solute concentration gradient.
ATP energy is used indirectly.
Phloem loading of sucrose
Experiments have shown that H+-ATPase, proton pumps, are located on the membrane of the
companion cells.
1. Sucrose is transported from the source cell into the companion cells using the
cotransport system (e.g. H+-ATPase, etc).
2. Once inside the companion cell, the sucrose moves into the sieve-tube elements
through plasmodesmata, direct cytoplasmic connections.
Phloem unloading of sucrose
The mechanisms for unloading sucrose varies according to the different types of sinks (e.g. root
cells, leave cells, etc.) in the plant.
In the leaves of some plants like sugar beets, sucrose flows along the concentration gradient
created between the high concentration in the sieve-tube elements through the companion cell
into the leave cell where sugar is rapidly used in cellular respiration and other cell activities
(passive transport).
In the roots of the sugar beet, sucrose moves passively from the sieve-tube elements to the
companion cells then into the root cell.
The root cell has an organelle that stores large amounts of sucrose. A proton pump located in
the membrane surrounding the organelle hydrolyzes ATP and brings sucrose into the organelle
by means of cotransport (active transport).
Chapter 33
PLANT NUTRITION
To make the molecules required for cells to function, plants must obtain a variety of elements.
About 60 naturally occurring elements have been found in plant tissues. Not all these 60
elements are considered essential for plant growth.
Plants require 16 essential elements for normal growth.
NUTRITIONAL REQUIREMENTS
Essential nutrients
1. Required for growth and reproduction. Without them the plant cannot survive and reproduce.
2. No other element can substitute for it.
3. It is required for a specific metabolic function. It is not just an aid in obtaining another
element.
Micronutrients are needed in trace quantities:

Iron, boron, manganese, copper, zinc, molybdenum and chlorine.
Macronutrients are required in large quantities:

Carbon, oxygen, nitrogen, hydrogen, potassium, phosphorus, sulfur, magnesium and
calcium.
Some examples of metabolic requirements:

Magnesium is needed for chlorophyll.

Potassium is an activator for over 40 enzymes, important in stomatal function and
contributes to osmosis causing the turgidity of the cell, and ionic balance.

Iron is a component of electron transport systems.

Calcium is an enzyme activator, involved in membrane permeability and in cell walls.

Iron, manganese, copper, zinc, and molybdenum are enzyme activators.
SOIL
Soil is the source of all macro and micronutrients except for C, H and O.
Soil is made of inorganic materials, organic matter, soil air, soil water and soil organisms.
Most soils are formed from rock (parent material) that is gradually broken down into smaller and
smaller particles by chemical, physical and biological processes called weathering.
1. Inorganic materials come from the weathered parent rock.
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Chemical and mechanical weathering.
Sand (0.02 - 2.0 mm), silt (0.002 - 0.02 mm) and clay (< 0.002 mm).
Particles larger than 2mm in diameter are called gravel and stone and are not
considered soil particles.
2. Organic matter consists of waste and the remains of organisms in different stages of
decomposition.
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Humus is the partially decayed organic portion of the soil.
Humus holds water and minerals.
Bacteria and fungi are the principal decomposers in the soil.
Organic matter adds nutrient to the soil and increases its water holding capacity.
3. Soil organisms form a complex ecosystem.
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Bacteria, fungi, algae, worms, insects, plant roots, mammals.
Organisms perform different functions: decomposition, aeration, and addition of nutrients
Nutrient availability
Many anions are very soluble and remain in solution in the ground water. they interact with the
water molecules forming H-bonds.
Anions are available to plants for absorption but they can be easily washed out of the soil by
rain or carry deep into the soil out of the reach of roots. This is called leaching.
Cations bind to the negative charges found in organic matter and on the surfaces of tiny clay
particles.
Each clay particle has a negative charge on its outer surface.
Clay particles have the greatest surface area and determine the fertility of the soil.
Organic matter and clay slows leaching.
Cations must go into solution before plants can absorb them.
MECHANISM OF NUTRIENT UPTAKE.
The plasma membrane is a bilayer of phospholipids with proteins embedded partially or all the
way through the bilayer.
Passive uptake
Some cations like K+ diffuse into the cell through protein channels.
The outside of the cell is more positive and the inside is more negative. K+ move from high
concentration to low concentration and from high positive to low positive area.
The combined effect of concentration and charge on an ion is called the electrochemical
gradient.
When the electrochemical gradient causes ions to move from the outside to the inside of the
cell, no energy is used and this flow of ions is called passive uptake or passive transport.
Root hairs have several nutrient-specific protein channels each for a different type of ion.
Active uptake
Active uptake requires the expenditure of energy in the form of ATP.
K+ can also be transported actively to the inside of the cell. K+ absorption also occurs when the
concentration of K+ is lower outside than inside the cell.
Experiments have shown that if the outside of the cell is acidic, K+ ions are transported more
readily.
A proton pump creates an excess of protons on the exterior of the root-hair membrane. then
the H+- K+ cotransport protein uses the resulting electrochemical gradient created by the
hydrogen ions to transport K+ to the inside of the cell against the concentration gradient.
Nutrient transfer via soil-dwelling fungi.
Mycorrhizae are mutualistic associations between roots and fungi.
The minerals move from fungus to root, the sugars from root to fungus.
Plants need to extract large quantities of P and N from the soil.
Fungi that grows in close association with the roots of plants help in the absorption of P and N.
The fungal hyphae (threads) can wrap around the root or can penetrate the root tissues.
This association of fungus and plant is mutually beneficial. It is called a mutualistic
relationship.
MECHANISMS FOR ION EXCLUSION.
Many elements can be harmful to plants. Some nutrients can be harmful is absorbed in large
quantities.
Tolerance to high concentration of certain ions vary from species to species and from population
to population within the species.
Passive exclusion
There is a possibility that salt-tolerant species have fewer channels and therefore absorb less
ions than salt intolerant species, which are capable of absorbing large quantities of ions.
Active exclusion
Metallothioneins are proteins that bind to metals. These bound metals cannot act as poisons
once bound to the metallothionein protein.
The gene that codes for metallothioneins is called MT2.
Plants that are tolerant of high ion concentration, e.g. copper ions, have a much higher
production of MT2mRNA.
This suggests that ion tolerance is a function of the metallothionein gene regulation.
NITROGEN FIXATION
Nitrogen, an element, is plentiful in the Earth's atmosphere. It exists there in the form of a
gas, with two nitrogen atoms bound together to form a molecule.
N2 is an extremely stable molecule that rarely reacts.
Some selected species of bacteria are capable of converting N2 to NH3 and use the energy
released for metabolic functions.
Nitrogen fixation is the process by which atmospheric N2 is reduced to NH3+ and made
available to produce amino acids and other nitrogen-containing organic compounds.
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Root nodules form when two types of bacteria, Rhizobium and Bradyrhizobium, infect the
roots of seedlings of leguminous plants (bean and pea family).
Rhizobium bacteria are often called rhizobia.
This infection is not harmful to the plant, and is not the source of a disease.
Once inside the plant, the rhizobia are called bacteroids.
The bacteroid enzyme nitrogenase to catalyze the fixation of atmospheric nitrogen, N2 into
ammonia, which may be utilized by the plant.
The enzyme nitrogenase contains metals, Mb, Fe, and S.
The bacteroids establish a mutualistic symbiotic partnership with the plant, which is
beneficial to the plant and the bacteria.
The plant provides energy-rich food, produced by photosynthesis, to the rhizobia. In return,
the rhizobia fix nitrogen for the plant.
Nitrogen-fixing bacteria and the colonization of the plant root.
Nod factors (for nodule formation) are synthesized and secreted by rhizobia when they detect
flavonoids released from leguminous plants.
Many flavonoids are yellow, orange, red, blue or black pigments found in flowers, leaves and
fruits of plants.
Flavonoids stimulate rhizobial cells to produce Nod factors. Nod factors contain sugars as part
of their molecular structure.
Each legume plant produces a different recognition flavonoid and each Rhizobium species
responds with one or more unique Nod factors.
Nod factors bind to proteins on the surface of the root hair cell membrane.
When Nod factors bind to the surface of the cell membrane, they set off a series of reactions
within the cell that leads to the transcription of some genes.
Bacteria digest (enzymatically) the cell walls.
The root hair ceases growth and instead deposit wall material at the invasion site.
Infection threads begin to form which are tubular ingrowths from the root hairs’ cell walls.
Infection threads are tubular structures formed by progressive inward growth of the root hair cell
wall from the sites of penetration.
Infection thread appears to be an invagination of the host cell’s membrane


Much dictyosome; i.e. Golgi ; activity in this area.
Cellulose is laid down on inner surface of the membrane (same as in cell wall
deposition)
Therefore, rhizobia never actually enter the epidermal (root hair or other) cell.
After the infection thread reaches the cortical cells, symbiotic bacteria are finally released into
the host cells of the nodule.
When the infection thread reaches a cell deep in the cortex, it bursts and the bacteria are
engulfed by endocytosis into endosomes. At this time the cell goes through several rounds of
mitosis - without cytokinesis - so the cell becomes polyploid.
Now they are bacteroids (have the special N-fixation enzymes).
Rhizobia induce cell division in regions of the cortex, which the bacteria enter by means of
branching infection threads.
The root nodule is a tumor-like growth that contains infected and uninfected cortical cells.
The symbiosis of rhizobia is highly specific. The bacterium causing a nodule in one species of
legume does not induce nodules in another species.
There are other plants that also form nitrogen-fixing mutualistic symbioses with bacteria.
NUTRITIONAL ADAPTATIONS OF PLANTS.
Most plants obtain K, P, and N from the soil either through passive transport or with the help of
mutualistic fungi.
In the tropic, however, there plants that do not follow this pattern.
Epiphytes are plants that grow on the branches or leaves of trees or other plants.
Epiphytes obtain their nutrients from rainwater that accumulates in crevices of the bark of trees
or in folds of the leaves.
Carnivorous plants make their own food through photosynthesis but they obtain their nitrogen
from animals they kill and digest.
Parasitic plants take food, water and nutrients from other plants.
There are about 3000 parasitic plants.
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
Holoparasites are not photosynthetic and obtain everything they need from the host
plant.
Hemiparasites are photosynthetic and make their own food, but obtain water and
nutrients from the host plant.
Mistletoe, a hemiparasite, has haustoria epiphytic roots that penetrate the living host tree.
Parasitism is more damaging than competition and lowers the total productivity (biomass
production) of the host plant.
Chapter 34
SENSORY SYSTEMS IN PLANTS.
Plants grow in a fixed location. If the environment becomes unfavorable, the plant must cope or
die.
Plants are capable of sensing environmental changes and make adjustments.
The ultimate control of plant growth and development is genetic.
Location of a cell in the plant body and environment influence gene expression in plants.
Chemical signals from adjacent cells may help the cell perceive its location in the plant body.
Environmental cues like changes in light and temperature influence gene expression.
Plant hormones are chemicals that are produce in one part of the plant and transported to
another part where they cause a physiological response.
TROPISMS
Tropism is growth response to an external stimulus from a specific direction.
Changes are permanent and irreversible.
Tropisms may be positive if the plant grows toward the stimulus or away from it.
Phototropism is a response to the direction of light.
Gravitropism (syn. geotropism) is a response to gravity.
Thigmotropism is a response to contact with a solid object.
SENSING LIGHT
Charles Darwin and his son Francis were the first to report on plants sensing light. They
conducted an experiment with the new shoots (coleoptiles) of canary grass.
They noted that the young shoots or coleoptiles bent toward the light: phototropism
An experiment conducted by Morgan and Smith showed that plants can sense infrared light.

Morgan and Smith controlled the amount of infrared light in their experiment because in
shade areas of the forest there is more infrared light than in open sunny places.
They found that...

Plants that are adapted to grow in open habitats elongate their stems more when they
are grown in the shade. This allows them to grow out of the shade into the sun.

Plants that grow in the forest floor do not elongate their stems as much when they are
grown in the shade, as those plants adapted to sunny habitats.
Other experiments have shown that lettuce seed germinate readily if they are exposed to red
light of 660 nm wavelength, but are inhibited if they are exposed to infrared light.

A lettuce seed germinating in the shade will be at a disadvantage.
Phytochromes
Sterling Hendricks and Vivian Toole proposed a mechanism to explain the sensing of light by
plants:

Phytochrome occurs in two forms: one form, Pr , absorbs red light at 660 nm and the other
form, Pfr, absorbs far-red light at 730 nm.

When either form absorbs its preferred wavelength, changes to the other form. They called
this phenomenon photoreversibility.

Pfr was considered to be the active form and Pr the inactive form of the phytochrome.
Butler and colleagues confirmed the existence of phytochrome by isolating it in 1959. They also
were able to change the color of the solution by exposing to red and infrared light, thus
confirming the change in molecular configuration of the pigment protein.
Subsequent studies have shown that in Arabidopsis thaliana there are five loci that code for five
phytochrome proteins.
All of these phytochromes appear to be photoreversible when exposed to red and infrared light.
These phytochromes cause different responses in the plant.
It is possible that there are more than five phytochromes in plants.
Phytochrome is also involved in the germination of seeds:


Exposure to red light converts Pr to Pfr and germination occurs.
Other physiological responses influenced by phytochrome include leaf abscission, pigment
formation in flowers and fruits, sleep movements, stem elongation, shade avoidance and
shoot dormancy.
Phytochromes monitor the amount of shade a plant receives.
Blue-light receptor: NPH1 protein.
Chlorophylls and carotenoids absorb blue light in the visible part of the spectrum during
photosynthesis.
Scientist theorized that there must be a blue-light receptor that triggers phototropism toward the
source of blue light.
Briggs and coworkers found a protein that is abundant in the membranes of cells in the tips of
emerging shoots. They hypothesized that this protein was involved in sensing blue light and
causing phototropism.
Christie and colleagues showed that Briggs' protein was in effect a photoreceptor of blue light.
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Many proteins switch from inactive to active when they receive a PO43+ from ATP.
Christie showed that NPH1 protein became autophosphorylated.
This autophosphorylation of NPH1 triggers the plant response toward blue light.
Several steps are involved between the sensing of a signal and the response of the organism.
This sequence of steps is called signal transduction.
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To transduce: to convert energy from one form to another.
A possible sequence of steps in transduction.
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Receptor changes configuration in response to signal, e.g. blue light.
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A second protein called a kinase found next to the receptor is in turn activated by the
receptor's change in configuration.
This change in the kinase converts the kinase into a catalyst that causes the
phosphorylation of a protein that starts the response by activating other proteins.
These newly activated proteins increase or decrease the transcription of certain genes or
alter the translation of some mRNA.
Genes called cry1 and cry2 are involved in the phototropic response.
This mechanism is not fully understood.
SENSING GRAVITY
Gravitropism (syn. geotropism) is a response to gravity.
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Gravitropism may be positive (toward) or negative (away from).
The curvature that occurs in reaction to gravity is due to differences in cell elongation
on the opposite sides of a root or shoot.
The molecule called auxin promotes cell elongation in shoot and inhibits it in roots.
The Statolith Hypothesis
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Statoliths are gravity sensors. In this case amyloplasts play the role of statoliths.
The perception of gravity is correlated with the sedimentation of amyloplasts,
starch-containing plastids, within specific cells of the shoot and root.
Amyloplasts accumulate at the bottom of cells in the root cap in response to gravity.
Pressure receptors in the amyloplasts' membrane become activated
The side of the cell opposite to the amyloplasts elongates.
The Gravitational Pressure or Hydrostatic Pressure Hypothesis
This theory proposes that ...
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Plants sense gravity by the hydrostatic pressure exerted by the protoplast on its cell wall.
There are receptor proteins in the cell membrane and extracellular matrix.
Pressure receptors at the top of the cell sense tension, while pressure receptors at the
bottom sense compression.
Amyloplasts are pulled to the bottom of cells by gravity and compress the receptors.
Transmembrane proteins.
The plasma membrane of animals and amyloplasts contain transmembrane proteins called
integrins.
Integrin span the membrane and project far into the extracellular space.
They bind to components of the extracellular matrix and the cytoskeleton inside the cell.
If these sites are altered by pressure, the change could indicate the direction of gravity.
The function of integrins is still to be elucidated. It may not play a role in gravitropism at all.
SENSING TOUCH
Sensors transduce the kinetic or mechanical energy of the signal or stimulus into chemical
energy.
When plants are buffeted by wind, the mechanical energy is transduced to a chemical response.
Electrical signaling.
The interior of plant cells has a negative charge relative to the exterior.
This occurs because proton pumps are active in many cells creating a charge separation across
the membrane.
This charge separation is called polarization.
This separation creates potential energy called a voltage. Potential energy is a tendency to
move.
Most plant cells then have a membrane voltage or a membrane potential.
The voltage across the membrane is measured with voltmeters.
Membrane potential are small and are expressed in units called millivolts, mV
By convention, membrane potentials are expressed as the state of a cell's interior relative to the
exterior.
Thus the resting potential of a plant cell - its normal state - is usually negative.
Action potentials
Plants like the Venus flytrap can send messages similar to nerve impulses.
This impulse is a drastic voltage change across the membrane due to a rapid flow of charges in
the form of ions, from the outside of the cell to the inside.
This rapid, temporary voltage change is called an action potential.
The action potential is a rapid change of the inside of the cell from negative to positive then
back to negative.
The resting potential of cells is about -70 mV.
Depolarization occurs when positive charges begin to flow into the cell lowering the membrane
potential by making the both sides more alike in charges.
The mechanical signal of pulling or touching causes the depolarization of the hair cells at the
base of the trap leaves of the Venus flytrap.
These cells swell with water and their pH increases dramatically. The mechanism involved in
this change in size is not well understood.
Chapter 35
COMMUNICATION: CHEMICAL SIGNALS
The tissues that sense environmental change are not necessarily those that respond to the
change.
Plant hormones are chemical messengers.
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Produced in one part of the plant.
Transported to another part of the plant.
Causes a physiological response: regulate growth and development.
Each hormone type causes several responses.
The responses of different hormones overlap.
There are five classes of plant hormones.
PHOTOTROPISM
The Darwins published their hypothesis about chemical signals and phototropism in 1881.
Peter Boysen-Jensen conclude in 1913, after conducting experiments, that the signal was
indeed a chemical and that it could diffuse from one part of the plant to another.
In 1925, Frits Went conducted his classical experiment with oat coleoptiles. Went was able to
collect the phototropic chemical in blocks of agar.
Went was able to produce a phototropic-like response without the stimulus of light.
Cholodny and Went proposed independently that the response is caused by an asymmetrical
distribution of the hormone.
Went named the hormone auxin from the Greek auxein, to increase.
The Cholodny-Went Hypothesis.
Auxin is produced in the tips of the coleoptiles.
The auxin is then transported from one side to another of the coleoptile in response to light.
Cells on the side with the greater concentration of auxin will elongate more causing the entire
stem to bend towards the light.
Other scientists have proposed that the auxin is destroyed in the side where the light strikes
causing a difference in auxin concentration along the stem.
Kögl and Thimann independently isolated the auxin hormone. It turned out to be indole acetic
acid or IAA.
The concentration of IAA is about 50 nanograms for every 50 grams of fresh tissue.
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1 ng = 1 billionth of a gram or 0.000 000 000 1 g
When IAA arrives at a target cell, then its message must be received and transduced to produce
the appropriate response.
Researchers found that IAA binds to a receptor protein called ABP1.
The Acid-Growth Hypothesis
It proposes that...
1. IAA produces or activates additional proton pumps.
2. The pumping of protons into the extracellular matrix causes K+ and other positive ions to
enter the cell.
3. This increase in solutes brings an influx of water into the cell.
4. There is then an increase in turgor pressure that makes cell expansion possible.
Hager and colleagues found that cells treated with addition IAA increased the number of proton
pumps by 80% relative to untreated control cells.
They also found that the acidity of the of the cell wall changed from a pH of 5.5 to one of 4.5.
The cell wall is rigid. So how does the cell wall expands?
Cosgrove found two classes of cell wall proteins that actively increase cell length when the pH
in the cell wall drops below 4.5.
These proteins are called expansins.
Expansins have been found in many species and tissues but how they work is not known yet.
One hypothesis proposes that these protein break the bonds between cellulose fibers and
pectin fibers or other wall components, allowing for stretching and expansion of the wall.
APICAL DOMINANCE
In apical dominance, the majority of the stem growth takes place in the apical meristem of the
shoot, and inhibits the growth of other meristems (e.g. lateral buds) located down the stem of
the plant.
Apical dominance occurs because auxin flows from the apex of the shoot down to the tissues
below.
Auxin transport is polar, unidirectional.
Radioactively labeled auxin molecules have shown that the hormones travel all the way down to
the central portion of the root, and when it reaches the root tip, the hormone moves out to the
epidermal cells and up for a short distance - the fountain model of flow.
The Cholodny-Went Hypothesis maintains that asymmetrical distribution of auxin causes
gravitropism in roots.
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In a horizontal root tip, gravity-sensing cells redistribute the auxin; more auxin goes to
the lower side of the root tip.
Asymmetrical auxin distribution causes the cell in the upper side to elongate causing the
root tip to bend downward - gravitropism.
The Chemiosmotic Model.
This model attempts to explain how polar transport takes place.
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The auxin in an acidic cell wall (pH 5.5) accepts a proton, H+, and becomes neutral.
There are influx carrier proteins located only on the upper side of the cell membrane.
Auxin is taken into the cell via this influx protein carrier that takes the auxin in with the
attached proton.
Inside the cell the pH is neutral (pH = 7) and the auxin loses the proton and becomes
negative, anionic.
There are carrier proteins specific for negative auxin called efflux carrier proteins,
located only on the cell membrane at the base of the cell.
Auxin leaves the cell through these carrier protein following an electrochemical gradient.
These events repeat and the auxin is transported from top of the cell to the bottom of the
cell and out to be pick up again by influx carrier of the cell below.
Gälweiler and colleagues identified a protein coded by the gene PIN1 that is located only at the
base of stem cells. It is hypothesized that these are the efflux carriers.
An overview of auxin action.
It is produce in the apical meristem of shoots, in young leaves and in seeds.
It is transported downward in parenchyma cells.
It causes cell elongation, promotes xylem and phloem differentiation, inhibits lateral bud
development, stimulates fruit development but delays ripening, and inhibits leaf abscission.
Auxin is the root-growth hormone sold in nurseries. It promotes root growth on cut-off shoots.
Because of its many effects on plants, some biologists have proposed that auxin overall function
is to signal where cells are in space, where the cell is located along the axis of the plant.
It helps to determine the overall shape of the plant due to changes in light availability, wind
strength, etc.
Auxin concentration signals how tissues should respond.
GROWTH AND DORMANCY
Abscisic acid (ABA) signals when the plant should stop growing.
Gibberillic acids signal when a plant should start growing again.
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There are more than 100 gibberillic acids identified but only a few have been shown
to act as hormones.
Seed dormancy and germination
Many plants produce seeds that must suffer a period of cold or hot temperatures, or drought
before a favorable period for growth arrives.
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Abscisic acid is the signal that inhibits seed germination.
Gibberillic acid signals the onset of embryonic development and germination.
The protein α-amylase is a digestive enzyme that breaks the bonds between the sugar subunits
in starch.
α-Amylase is release from the aleurone layer of the seed during germination.
Gibberillic acid (GA) activates the production of α-amylase.
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The embryo absorbs water and triggers the diffusion of GA.
GA produced by the embryo reaches the membranes of cells in the aleurone layer.
Receptors in the membrane receive this signal.
The receptors activate the production of Myb, a transcription activator.
Myb travels to the nucleus, binds to the α-amylase promoter, and triggers transcription
and α-amylase production.
The Myb protein that binds to the promoter has been named GAMyb.
Abscisic acid decreases the production of α-amylase.
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Preliminary data suggests that ABA induces the production of Myb proteins that act as
gene repressors.
Myb proteins bind to the α-amylase promoter and shut down amylase production.
ABA activates transcription repressors.
Both activators and repressors compete for the same site in the promoter gene.
If ABA is in higher concentration, repression dominates and dormancy occurs.
If GA is in higher concentration, activators dominate and germination proceeds.
Therefore...
1. A cell's response to a hormone often occurs because specific genes are turned on or off.
2. Hormones rarely act on DNA directly.
3. Different hormones interact at the molecular level because they induce different transcription
activators and repressors. The relative amount of these regulatory factors determines which
activators or repressors dominate the response.
Closing guard cells
Guard cells open in response to light in order to allow carbon dioxide to enter the mesophyll
spaces for photosynthesis.
In cases of drought, the roots cannot obtain enough water and the stomata close.
Experiments have shown that plants under water stress have a higher concentration of ABA in
their roots and leaves.
Shoot elongation
Japanese scientist isolated a substance in the 1930s that causes rice seedlings to elongate
abnormally and fall over before harvest.
These rice plants were infected with the fungus Gibberella fujikuroi. Treating seedling with
extract of the fungus caused abnormally long plants.
Analyzing stem-length mutants
One of the traits studied by Gregor Mendel was dwarfism in peas.
Mendel found that one allele coded for normal stem length, tall, and another for dwarf stems.
Dwarf pea plants attain normal height when treated with GA1. This suggests that plants can
react normally but cannot manufacture gibberellin.
Researches found that a locus called Le (for length) was responsible for the synthesis of GA1.
Diane Lester found a locus that encodes for the enzyme 3β-hydroxylase.
3β-hydroxylase catalyzes the addition of -OH to a gibberellin called GA20 and converts it to GA1.
Lester found that in mutant DNA, the Le gene coded for the AA threonine instead of alanine.
This produces an abnormal enzyme that cannot add -OH to GA20 to form the active gibberellin
GA1.
Role of gibberellins in plant growth
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It is produced in young leaves, roots, shoot apical meristem and in the seed embryo.
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Method of transport in the plant is unknown.
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It promotes seed germination, cell division and elongation, fruit development, flowering
in some plants and breaks seed dormancy and winter dormancy.
OTHER HORMONES
1. CYTOKININS
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They are produced in the roots.
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Travel upward in the xylem.
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Promote cell division and differentiation in which unspecialized cells become specialized,
promotes chloroplast development, stimulates lateral bud development, inhibits
abscission and delays senescence.
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There is evidence that cytokinins activate genes that keep the cell cycle going.
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Zeatin was the first isolated naturally occurring cytokinin.
2. ETHYLENE
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It is a gaseous hormone produced in stem nodes, aging tissues and ripening fruits.
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It probably diffuses out of the tissue that produces it.
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It promotes ripening of the fruits, senescence and abscission, inhibits cell elongation,
stimulates germination of seeds and it is involved in responses to wounds and infections
by microorganisms.
OTHER CHEMICALS INVOLVED IN PLANT GROWTH AND DEVELOPMENT.
Polyamines are organic molecules with two or more amine groups (- NH2).
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They may be involved in gene expression and increase the transcription of DNA and
translation of mRNA.
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They exist in high concentration in plant tissues and are not transported extensively through
the plant.
Systemin, a plant polypeptide, stimulates plant defenses that produce chemicals that disrupt
insect digestion.
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Present in very small quantities.
Oligosaccharins are cell-wall fragments consisting of short, branched chains of sugar residues.
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Present in quantities lower than hormones.
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Bind to membrane receptors and affect gene expression and have many effects on plants.
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Some have antibiotic properties and kill fungi and other plant pathogens.
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Others promote vegetative growth and inhibit flowering.
Salicylic acid help to defend the plant against insect attack and promotes wound healing.
The function of brassinosteroid hormones in plants is not clear. One steroid hormone seems
to be involved in light regulated developmental steps, others in growth. Plants without these
hormones are dwarf.
Chapter 36
PLANT REPRODUCTION
Only one group of plants produces flowers, the angiosperms.
Bryophytes (mosses, liverworts, etc.), pteridophytes (ferns, ground pines), gymnosperms
(conifers, ginkgo, etc.) do not produce flowers.
PLANT REPRODUCTION - AN INTRODUCTION
Flowers are a collection of organs designed to produce gametes, attract gametes, and develop
seeds.
Sexual reproduction
Most plants reproduce sexually.
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Meiosis: reduction in the number of chromosomes.
Gametogenesis: Formation of sperm and eggs.
Fertilization: fusion of sperm and egg.
Flowers having both male and female organs are called perfect.
Flowers having either male or female organs are called imperfect.
Many plants are capable of outcrossing, that is, they exchange gametes with other plants of
the same species.
Many plants are also capable of self-fertilization.
Plant life cycle
Plants have a characteristic life cycle called alternation of generations.
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Sporophyte generation is diploid, 2n, and produces haploid spores through meiosis.
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Gametophyte generation is haploid, n, and produces haploid gametes through mitosis.
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Fertilization restores the diploid stage; meiosis restores the haploid stage.
Asexual reproduction.
Offspring are formed without the fusion of gametes.
Offspring are genetically similar to the parent plant.
Stems, leaves and roots may be adapted to asexual reproduction.
Apomixis is the formation of seeds without fertilization, akin to parthenogenesis is animals.
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The new generation can come from an unfertilized ovule or from a vegetative cell.
Modified stems may give rise to independent plants in time:
Rhizomes are underground horizontal stems that may or may not be fleshy.
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They produce new plants by fragmentation of the old rhizome.
Tubers are fleshy underground stems enlarge for food storage.
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Tubers produce independent plants once the parent plant dies.
Bulbs are modified underground buds attached to short stems with storage leaves.
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It frequently forms axillary buds that separate and grow into independent plants.
Corms are short, erect underground stems that store food in their tissues and are covered with
papery leaves.
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Small axillary buds give rise to new corms.
Stolons or runners are horizontal, aboveground stems.
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They produce buds that give rise to small plants that root and become independent.
Modified leaves can produce plantlets that break off and give rise to new plants.
REPRODUCTIVE STRUCTURES
Different groups of plants have their own characteristic male and female reproductive structures.
Angiosperms are the most abundant group with about 230,000 species.
Flower formation begins in the apical meristem.
Flower parts are modified leaves. They differentiate under the influence of environmental
signals.
Day length, hormones and other cues.
Photoperiod is the length of daylight in a 24-hour day.
Short-day plants (long-night plants) flower when the night length is equal to or greater than
some critical period.
 Plant detects the shortening of the day or lengthening of the night.
 Minimum critical night length varies with the species.
 Fall flowers like poinsettias and chrysanthemums.
Long-day plants (short-night plants) flower when the night length is equal to or less than
some critical period.
 Plant detects the lengthening of the day and shortening of the night.
 Maximum critical night length varies with the species.
 Spring flowers.
Plants in the tropics are not sensitive to day length because the day length variation is minimal.
Desert plants are sensitive to watering, the rainy season, rather than day length.
When light, water and nutrients are present in favorable amounts, the plant may begin to flower
before the arrival of the correct day length.
Gibberellins or other hormones could trigger the production of reproductive organs.
Genes respond to the signal to flower.
Researchers have confirmed that different genes are responsible for the response to different
flowering signals.
Blazquez and Wigel found that genes controlling flower development have promoters with
binding sites for more than one type of transcription activator.
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Day length and hormones (GA)
Structure of the flower.
The flower is a modified branch apex, and is involved in sexual reproduction.
Reproductive and accessory organs are normally arranged in whorls or circles of structures:
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Sepals, petals stamens, and carpels.
The whorls of organs sit on an enlarged branch end called the receptacle.
The sepals form the calyx and protect the flower bud.
The petals form the corolla and attract animals to assist in pollination.
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Petals may or may not be present.
Nectaries may be located at the base of petals and contribute to pollination.
The stamens are the male reproductive organs.
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They consist of a filament and an anther.
Pollen grains form in the anthers.
Each pollen grain contains two cells; one produces two sperm nuclei, and the other
produces a pollen tube to transfer the sperm nuclei to the ovule.
A pollen grain represents a male gametophyte.
The carpels are the female reproductive structures.
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A flower may have one or several carpels.
Carpels may be separate or fused.
Carpel usually has a style and stigma.
Ovary is another name for the lower portion of the carpel.
An ovary may be formed by various fused carpels.
Pistil is another name for the female reproductive structure.
A pistil may be formed by a single carpel or by several fused carpels.
The ovary contains one or several ovules.
The ovule produces and contains the embryo sac.
The embryo sac produces two polar nuclei and one egg.
The egg and the polar nuclei are involved in the process of double fertilization.
THE ANGIOSPERM LIFE CYCLE
Producing the female gametophyte.
OVULE → MEGASPORE → MEGAGAMETOPHYTE (EMBRYO SAC) → EGG
1. The ovule consists of an inner tissue called the nucellus and one or two protective layers
called integuments.
2. The integuments form one small opening at one end of the ovule, the micropyle.
3. The one diploid cell in the nucellus produces four haploid cells or megaspores through
meiosis.
4. Three nuclei degenerate and one haploid nucleus remains.
5. The functional megaspore enlarges at the expense of the other cells of the nucellus.
6. This nucleus inside the cell divides mitotically twice, forming first four nuclei and then eight
nuclei.
7. The eight nuclei separate and form six small cells and one large cell, the megagametophyte
or female gametophyte.
8. Three cells migrate towards the micropylar end, one cell becomes the egg and the two
others are called synergids eventually degenerate.
9. Three other cells migrate to the end opposite to the micropyle.
10. Two nuclei remain the in the center of the large cell. These two nuclei are called the polar
nuclei.
11. At this point this structure is called the female gametophyte or megagametophyte or
embryo sac.
Producing the male gametophyte
ANTHER → SPOROGENOUS TISSUE → MICROSPORE → MICROGAMETOPHYTE (POLLEN GRAIN) →
GENERATIVE CELL → MALE NUCLEI
1. The anther and the filament make the male reproductive organ of the plant.
2. Inside the anther, the diploid cells of the sporogenous tissue divides meiotically to produce
haploid microspores.
3. Each diploid cell produces four haploid microspores.
4. Each of the four cells forming called now microspores, divide mitotically to form pollen grain
made of generative cell enclosed in a larger vegetative cell.
5. A very resistant outer layer of sporopollenin called the exine, and an inner layer of pectin
called the intine surrounds this two-cell structure or pollen grain.
POLLINATION AND FERTILIZATION
Pollination and fertilization are two different things.
Fertilization occurs in all plant groups, from mosses to flowering plants.
Pollination occurs in gymnosperms and in angiosperms.
Pollination
Pollination is the transfer of pollen from the anther to the stigma.
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Plants can self-pollinate or cross-pollinate.
During pollination the male and female gametophytes are brought together.
Animals or wind usually carry pollen (male gametophytes) to the stigma of the flower.
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Water is the medium of pollen transport in some specialized cases.
Flowering plants and their animal pollinators have evolved together. This is mutualistic
relationship between the plant and the animal.
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Pollinators use nectar and pollen as sources of energy and protein.
Plants are pollinated and outcrossing can take place.
Some plant species deceive the pollinators and do not provide food.
Mosses and their relatives, and pteridophytes do not form pollen are the first groups to evolve.
Gymnosperms form pollen and are wind pollinated.
Angiosperms are the last group of plants to evolve and are mostly animal and wind pollinated.
Wind and animal pollination eliminates the need of water for sexual reproduction and allows
plants to colonize upland environments away from water.
Pollination by animals can be a very precise process and a more efficient process.
Evolutionary changes in animals affect plant populations and vice versa. Angiosperms and
animal pollinators are an example of coevolution.
It is estimated that insects pollinate about 70% of flowering plants.
About 30% of our food come from crops pollinated by bees.
Plants pollinated by wind often have reduced or absent petals, produce large amounts of pollen
and do not have scent.
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Insect-pollinated flowers often have yellow or blue petals and have scent.
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Bird-pollinated flowers are often yellow, red or orange, and do not have scent.
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Bat-pollinated flowers are creamy white and have strong scent.
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Fly-pollinated flowers often smell like decaying flesh.
Self-incompatibility
Some plants can self-fertilize if pollen from the same plant fall on the stigma.
Other plants cannot self-fertilize: these plants are self-incompatible.
There are several mechanisms involved in the compatibility or not of the pollen and the stigma.
The S locus in the cabbage family:
1. The "S locus" consists in reality of three loci.
2. There are multiple alleles of these genes, up to 50.
3. The proteins coded by these loci are located one in the membrane of the stigma cells,
another in the cell wall of the stigma cells, and the third is secreted by mature pollen
grains.
4. If the proteins secreted by the pollen are the same as one or both of the proteins in the
cell membrane and wall of the stigma, the pollen grain does not form a pollen tube.
Similarity of alleles means that they are probable from the same plant.
5. If the proteins secreted by the pollen tube are different from both of those in the cell wall
and the cell membrane of the stigma, the pollen tube forms. The pollen comes from a
different plant.
Fertilization
Fertilization is the fusion of gametes.
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It restores the diploid condition in the zygote.
The male gametophyte produces a long tube that grows through the stigma, the style and
enters the micropyle of the female gametophyte (syn. embryo sac).
The generative cell of the pollen divides to form two sperm nuclei that move into the pollen tube.
Double fertilization is a unique phenomenon that occurs in angiosperms only.
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Egg and one sperm form the zygote, 2n.
The two polar nuclei and the second sperm form the endosperm, 3n.
The endosperm stores food for the developing embryo.
The ovule will develop into a seed and the ovary will develop into a fruit.
THE SEED
The zygote is the first cell of the sporophyte.
As the seed matures inside the ovary, the ovule and the embryo become surrounded by a tough
seed coat.
At the same time, the ovary develops into a fruit that protects the seeds and aids in their
dispersal.
Embryogenesis
Embryonic development follows a pattern:
The zygote divides into two cells; the upper cell develops into the embryo proper, and basal cell
develops into a column of cells that brings in nutrients and gibberellins to the developing
embryo.
The suspensor is short lived and undergoes a process of programmed cell death or apoptosis.
Proembryo  globular embryo  heart-shaped embryo   mature embryo.
A mature embryo consists of a radicle, hypocotyl, one or two cotyledons and the plumule.
Radicle: embryonic root.
Hypocotyl: embryonic or seed stem.
Cotyledons: embryonic or seed leaves.
Plumule: embryonic bud or meristem.
Seed maturation
The mature seed contains the embryo and the nutritive tissue (endosperm or cotyledons).
During maturation seeds undergo a drying process.
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Water makes more than 80% of the plant cell but only 5 - 20% of the seed cells.
Germination will occur only when the seed absorbs water, when water is available in the
environment.
Sugars seem to be involved in maintaining the stability of the cell membranes in seeds by
forming a viscous sugary material with little or no water.
When water becomes available, the sugary viscous liquid dissolves and germination starts.
Fruit development and seed dispersal.
Seeds are enclosed in fruits.
Fruits are ripened ovaries and may or may not include other parts of the flower.
The ovary wall or pericarp thickens during maturation.
Fruit classification
1. Simple fruits develop from a single pistil.
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
One or several carpels involved.
May be fleshy (e.g. berries) or dry (e.g. grains).
2. Aggregate fruits develop from a single flower with many separate ovaries (e.g.
blackberries, gumballs).
3. Multiple fruits develop from a many flowers growing closely together on a common axis
(e.g. pineapple).
4. Accessory fruits develop from tissues other than the ovary (e.g. strawberry, apples, and
pears).
Seeds and fruits are adapted to various means of dispersal, including wind, water, animals and
explosive dehiscence.
Dry simple fruits are classified as dehiscent (to break open) or indehiscent (not break open)
Most dry fruits are dispersed by the wind or fall to the ground. Some dry fruits have hooks that
attach to the fur of animals or clothes and are transported far away.
Animals are the most common dispersal agents of fleshy fruits. The seeds pass through their
digestive track unharmed and germinate after the animal defecates at a different location.
Seed dormancy
Seed dormancy is characteristic of plants that live in seasonal climates.
There is not one single mechanism involved in seed dormancy.
In some plants, dormant seeds have high concentration of abscisic acid (ABA).
When the dormant seeds are exposed to water, the abscisic acid washes out and germination
starts.
Some seeds have a very thick and resistant coat that does not allow water or oxygen to reach
the embryo. These seeds must be scarified by abrasion, fire or passage through the digestive
track of an animal.
Other seeds must undergo a period of cool or freezing weather before germination.
Seed germination
During germination, glucose breakdown may be entirely anaerobic.
Three phases have been distinguished in seed germination:
Phase 1:
Phase 2:
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Increase in water uptake, oxygen consumption, and protein production.
No new mRNA is transcribed at this point.
The processes are controlled by mRNA stored in the seed.
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Water uptake stops.
Newly transcribe mRNA is translated into proteins.
Mitochondria begin to multiply.
Proteins are manufactured to support growth.
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Water imbibition is resumed.
Cells enlarge and the embryo burst through the seed coat.
Radicle emerges and begins to penetrate into the soil.
Hypocotyl elongates.
Phase 3:
PLANT DEFENSE SYSTEMS
Chapter 37
Plants, like animals, are subject to diseases and a type of predation called herbivory.
Plants cannot escape threat like animals do; plants are sessile.
There is great deal of research going in the area of plant defenses.
BARRIERS TO ENTRY
There are morphological and anatomical adaptations that protect plants against pathogens and
herbivores.
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Pathogen: a disease causing organism.
Herbivores: plant eaters; this definition includes animals that feed on nectar, fruit,
seeds, etc. and not only that feed on leaves.
Cuticle and cutinase.
The cuticle of plants is made of wax.
Epidermal cells secrete the cuticle.
Cuticular waxes consist of a matrix of long lipid chains in which cutin, another lipid, is
embedded.
The lipid chains are cross-linked.
Waxes are hydrophobic and protect plants against loss or entry of water through the waxy
area.
The waxy cuticle forms the first line of defenses against the entry of pathogens. It prevents
viruses, bacteria, spores, fungi, etc. from penetrating into the body of the plant.
Wounds in the plant allow the entry of pathogens.
Some fungi have cutinase enzymes that disrupt the cross-links in the lipid chains and penetrate
into the plant.
There are different types of cutinase enzymes with different ability to disrupt the cuticle matrix.
The ability to disrupt the matrix accounts for the virulence of the pathogen.
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Virulence: ability to cause disease.
In some fungi, virulence depends on the number of genes for cutinase present in the strain.
MORPHOLOGICAL ADAPTATIONS (WEAPONS)
Spines, thorns, prickles, and hairs are adaptations to discourage herbivores from eating plant
parts.
Associations with animals also deter herbivores, e.g.
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Ants and acacias: the acacia has a swollen and hollow thorn base provides a home
for the protective ants.
Some ferns have swollen rhizomes in which ants live.
PLANT POISONS
Chemical defenses include the accumulation of compounds that make the plant parts difficult
to eat, hard to digest, unpalatable, repugnant or toxic.
Plant defense compounds resemble molecules required by metabolic functions, e.g. amino
acids. It is hypothesized that mutations caused chemicals that were not functional in metabolism
but had a repellent property that allowed individuals with the mutation to survive and reproduce
more successfully than those without the mutations.
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E. g. The secondary metabolite DIMBOA has been shown to be an offshoot of tryptophan
synthesis involving five different genes that synthesize an intermediate product that led to
the final production of DIMBOA.
Protective chemicals are called secondary compounds since they are not essential for the
metabolic processes of the plant.
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Substances not produced as part of primary metabolism in plant; frequently with an
uncertain function.
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Plant poisons or allelochemics are constantly produced in plants. There is no need for an
stimulus.
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Allelochemics are secondary substances capable of modifying the growth, behavior or
population dynamics of other species through inhibitory or regulatory processes.
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These compounds cover a wide range of organic chemicals: toxic proteins, tannins,
terpenes, alkaloids, phenolics, resins, steroidal, cyanogenic and mustard oil glycosides and
tannins (contain aromatic rings, some are glycosides).
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Tannins bind to the digestive enzymes of insects that sicken the insect. They also interfere
with protein break down.
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Phenolics are very common amino acid derivatives found in seed-producing plants; they are
the burning substances in poison ivy and poison oak. Alkaloids are also amino acid
derivatives found thousands of species of plants. Cyanogenic glycosides are found in a few
hundreds of species.
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Glycosides are oligosaccharides bound to alcohols, phenols or amino groups. They usually
interfere with the formation of ATP.
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Nicotine, caffeine, cocaine and morphine are alkaloids. Alkaloids are found in about 20% of
the plant species. Alkaloids are highly toxic to herbivores and parasites; disrupt several cell
mechanisms: enzyme poisoning, inhibition of protein synthesis, disruption of a membrane
transport system, etc.
THE COST OF DEFENSE
Plants spend energy resources in making secondary compounds, e. g. large amount of ATP.
The number of parasites and herbivores play a selective role in the plant population, e. g...
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Abundant herbivores will eliminate poorly defended plants and well-defended plants will
survive and produce offspring that synthesize large amount of secondary metabolites.
Well-defended plants do better when pest pressure is high.
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Few herbivores will allow poorly defended plant to survive in large numbers and the
population will have many individuals that synthesize small amounts of poison.
Producing secondary metabolites reduces the amount of energy available for growth and
reproduction.
Responding to pathogens.
Plants can respond to pathogens and herbivores after they are attacked.
Infected cells respond by dying. This is called a hypersensitive response or HR.
Gene-for-gene hypothesis
When gene products from the plant and the pathogen match and interact.
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Pathogens infect the plant via a wound or some other means.
Pathogens release their own proteins in the plant tissues.
These proteins cause the plant to react and produce their own proteins that may or may
not inactivate the pathogen's proteins.
Binding between the plant and pathogen proteins causes the hypersensitive response
and the plant cell dies and the pathogen with it.
Not binding (no match) between the plant and the pathogen proteins causes nor HR
reaction and the plant becomes seriously infected and eventually succumbs to disease.
Experiments from around the world have confirmed the gene-for-gene hypothesis through the
synthesis of R (plant gene) and avr (virulent/avirulent pathogen gene) gene products that
interact.
These experiments were confirmed in 1996 by an experiment designed and carried out by
Scofield and colleagues.
Resistance loci.
When similar genes are found clustered together in a chromosome they are said to form a gene
family.
Gene families originated in all probability by incorrect crossing over or some other geneduplicating event.
Diploid plants have alleles for the same locus, one in each chromosome. If there are many loci,
there are also many alleles, which are probably different from one another.
The different alleles allow to recognize different protein from the same pathogen. This is
important because avr products (virulent) arise continuously in the pathogen population via
mutations.
This large number of R alleles allows the plant to react to a wide variety of pathogenic products
and therefore create resistance.
Reactive oxygen intermediates (ROI)
The interaction between R proteins in plants a pathogen's proteins initiate a series of plant
responses that include the formation of hydrogen peroxide, H2O2, and O2-, superoxide. These
molecular responses are collectively called ROI or reactive oxygen intermediates.
ROI create reactions that strengthens the cell wall, trigger plant cell death or that of the
pathogen.
Researchers have found that ROI molecules and nitric oxide, NO, must be present in order to
have the typical HR.
Phytoalexin production
Plants can produce certain antibiotic compounds called phytoalexins.
A phytoalexin is small molecule that is induced by infection and that poisons the pathogen.
Plants make these antibiotics when infected by a pathogen, e. g. chickpeas.
Phytoalexins occur at the point of infection but a slower and more widespread reaction occurs,
the systemic acquired resistance (SAR).
Salicylic acid concentration increases dramatically in infected plants. Experiments have shown
that addition of SA triggers an SAR response.
It is not clear if SA is the hormone that causes SAR or is only a local signal that causes the
expression of genes involved in the SAR response.
Responding to herbivores
Most species of insects feed on leaves, stem and root parts, pollen, etc. They are herbivores.
Proteinase inhibitors.
Proteinase inhibitors inhibit the enzymes responsible for the digestion of proteins.
Herbivores detect proteinase inhibitors by taste and avoid plants with large concentration these
substances.
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The hormone systemin is a polypeptide 18 AA long.
Damaged cells in potato and tomato plants produce systemin, which binds to receptors of
undamaged cells.
This activated receptors produce a series of reactions that eventually produce jasmonic
acid.
Jasmonic acid triggers the transcription of more than 15 gene products including proteinase
inhibitors that will discourage herbivores from attacking this plant.
Parasitoids
Parasitoids lay their eggs in the larvae of insects and devour the larva slowly as they grow and
develop. By the time larva dies, the parasitoid larvae is ready to emerge as an adult.
Caterpillar saliva has a substance called volicitin that induces damaged leaves to produce
volatile substances that attract wasps.
These wasps are parasitoids and lay their eggs in the caterpillars that have damaged the plant.
In this way plants recruit parasitoids to infect the herbivores that are eating them.